1. Technical Field of the Invention
The present invention relates generally to spark plugs and their mounting in cylinder heads of engines. More particularly, the invention relates to a spark plug and cylinder head assembly which can be made at low cost and ensure reliable ignition of the air/fuel mixture in a combustion chamber of an engine.
2. Description of the Related Art
In an internal combustion engine of a motor vehicle, there is mounted, in a cylinder head of the engine, a spark plug for igniting the air/fuel mixture in a combustion chamber of the engine.
The spark plug includes a center electrode and a ground electrode, and discharges sparks across a spark gap formed between the center and ground electrodes. The discharged sparks then causes the formation of a flame core, and the flame grows around the flame core to ignite the air/fuel mixture.
However, when the flame core is formed too close to an inside surface of the cylinder head which faces the combustion chamber, the flame core will be cooled by the inside surface, hindering the growth of the flame.
To solve the above problem, one may consider locating the spark gap of the spark plug deep into the combustion chamber. However, in this case, the temperature of the ground electrode will become too high, causing a pre-ignition of the air/fuel mixture.
Japanese Utility Model Publication No. H5-87274 discloses a spark plug and cylinder head assembly which is made by mounting a spark plug in a cylinder head of an engine. In this assembly, the spark plug includes an air pocket that is formed between a metal shell and an insulator of the spark plug and opens to a combustion chamber defined by the cylinder head. A communication hole is formed in the metal shell to extend between the air pocket and an outer side surface of the metal shell. Further, a communication path is formed in the cylinder head to fluidically connect the air pocket of the spark plug to the combustion chamber. In operation, a flow of the air/fuel mixture is induced through both the communication path and the communication hole, expelling the air/fuel mixture having entered the air pocket of the spark plug to the combustion chamber.
The above spark plug and cylinder head assembly may be effective in shifting sparks induced in the spark gap of the spark plug away from the inside surface of the combustion chamber.
However, to make the spark plug and cylinder head assembly, it is necessary to form the communication hole in the metal shell of the spark plug and the communication path in the cylinder head and to accurately align the communication hole with the communication path. Consequently, both the manufacturing and assembly processes of the spark plug and the cylinder head become complicated, increasing the cost of the resultant spark plug and cylinder head assembly.